PROJECT SUMMARY Clinical and epidemiological evidence suggests that ~40% of patients with ductal carcinoma in situ (DCIS) left untreated will progress to invasive breast cancer. However, due to our inability to distinguish lesions that will progress to invasive cancer from those that will remain non-invasive indefinitely, all DCIS patients are treated with surgery with or without radiation. Unfortunately, very little is known about how pre-invasive DCIS cells acquire the ability to invade the adjacent stroma, and how the stroma influences localized invasion has not been studied. The proposed studies aim to define the molecular interactions between infiltrating macrophages and pre-invasive epithelial cells, and uncover the mechanisms that promote tumor development in early stage lesions. The central hypothesis is that distinct macrophage populations promote the transition to invasive cancer and that Axl-mediated invasion is dependent on the transcription factor C/EBP?. This hypothesis will be tested by the following aims: Aim 1: To track macrophages in vivo during the transition from pre-invasive to invasive cancer. Aim 2: To characterize macrophage heterogeneity in early stage lesions. Aim 3: To determine whether C/EBP?-induced Axl in pre-invasive cells is required for localized invasion in early stage lesions. Our studies will utilize several mouse models and 2-photon microscopy to uncover the mechanism of localized invasion. Significance: The proposed studies will identify a mechanism by which DCIS cells proliferate and/or invade the basement membrane, and define how the stroma promotes progression to invasive cancer, addressing major knowledge gaps in the field of premalignancy.